Flexible circuits are circuits that are formed on flexible dielectric substrates such as polymeric materials. The circuits may have one or more conductive layers as well as circuitry on one of the major surfaces or on both major surfaces. The circuits often include additional functional layers, e.g., insulative layers, adhesive layers, encapsulating layers, stiffening layers and the like. Flexible circuits are typically useful for electronic packages where flexibility, weight control and the like are important. In many high volume situations, flexible circuits also provide cost advantages associated with efficiency of the manufacturing processes employed.
U.S. Pat. No. 4,914,551 discloses circuits as provided on flexible dielectric materials. U.S. Pat. No. 4,231,154 discloses a flexible circuit with conductive traces on one or both major surfaces. U.S. Pat. No. 4,480,288 discloses flexible circuits with circuitry on both major surfaces. U.S. Pat. No. 5,401,913 discloses a multi-layer flexible circuit including multiple flexible circuits stacked relative to one another and interconnected using metallized through-holes commonly referred to as vias. All of these references disclose various aspects of flexible circuits, but none make mention of the benefits of utilizing diamond-like carbon or materials providing diamond-like carbon properties to enhance the static charge dissipative characteristics and performance of flexible circuits.
Various types of flexible circuits are known in the industry. Generally speaking, the key differences in the various circuits stem from a number of design requirements for the devices that the circuit is connecting together, along with the requirements and limitations of the processing methods used to make the circuit. Typically, the flexible circuit is connecting a semiconductor device of some sort (integrated circuit, microprocessors, or the like) to another flex circuit, a rigid circuit board or a component of a device. The design factors associated with items the circuit is connecting include, but are not limited to, the number of input and output (I/O) leads from a semiconductor device that needs to be connected; the means and process for interconnection of the flexible circuits to another circuit or to a device; the required size and weight of the finished product; the environmental conditions under which the circuit will be assembled and used; and the data transmission rates to which the circuit will be subjected. All of these design factors as well as the methods and equipment used to manufacture the circuit will at least partially determine circuit design parameters. Circuit design parameters include whether the means for interconnecting the flexible circuit to another item is a Ball Grid Array (BGA), an array of bonding pads, or series of discrete leads; whether the circuit has one or more conductive layers, and if so, on one or both sides; if the materials need to be chemically stable to prevent outgassing; or if they need to be compatible for use and assembly at elevated temperature and humidity levels.
Diamond coatings, diamond-like carbon films, and uses for them are also known. These coatings and films possess a number of desirable properties, including high hardness, optical clarity, low friction, high thermal conductivity, high dielectric constant, high chemical stability, low gas and vapor permeability, and other properties. Furthermore, the composition of diamond-like carbon can be modified to control the measured value for many of these properties. The typical properties for diamond-like carbon are presented in the table below.
Typical Properties for Diamond-Like Carbon Density, g/cm.sup.3 1.6-3.0 Hardness, Vickers, kg/mm.sup.2 2000-9000 Young's Modulus, Gpa 100-200 Dielectric Constant 8-12 (between 45MHz and 20 GHz) Electrical Resistivity, ohms/.quadrature. 10.sup.5 -10.sup.15 Excitation Coefficient .001-0.5 (between 200 and 1000 nm) Index of Refraction @ 10 .mu.m 1.8-2.4 Optical Band-Gap 0.8-3.0 eV Thermal Conductivity @ 25 C., W/m-K 400-1000
Diamond-like carbon coatings and films may be formed and deposited by processes using hydrocarbon or carbon sources. Carbon source deposition methods include ion beam sputter deposition, DC magnetron sputter deposition, pulsed cathodic arc deposition, laser ablation deposition, and ion beam assisted carbon evaporation. Hydrocarbon source deposition methods include ion beam, microwave plasma, directed plasma discharge, various types of plasma-assisted chemical deposition methods, radio frequency plasma deposition, cathodic arc deposition, and the like. U.S. Pat. Nos. 4,698,256; 4,400,410; 4,383,728; 4,504,410; 4,746,538; 5,643,343; and 7,707,409 disclose known processes for producing diamond-like carbon films and coatings.
Diamond and diamond-like carbon films are currently being used in a variety of applications. These applications include eyeglasses, semiconductor devices, drilling and machining tools, beverage containers, and many other applications requiring one or more of the properties provided by diamond-like carbon. U.S. Pat. No. 5,508,071 discloses an annular interior surface having a layer of diamond coating for improved abrasion resistance. The coating is deposited on substrates such as metal, alloys, and ceramics. Because chemical vapor deposition (CVD) of diamond layers takes place at very high temperatures, it cannot be used for many polymeric substrates such as polyimide that will degrade at the elevated diamond-forming temperatures. Further, the polycrystalline nature of CVD diamond dictates a very hard, brittle coating with little flexibility.
The term "diamond-like carbon" is typically applied to noncrystalline materials, especially those in which tetrahedral diamond bonds predominate. U.S. Pat. No. 4,576,964 discloses barrier films made from flexible polymeric substrates having amorphous carbon coatings adhering thereto. U.S. Pat. No. 5,508,092 discloses optically transparent abrasion wear resistant coated substrates comprising a parent substrate, one or more interlayers and a top coating of diamond-like carbon or other low-friction material. U.S. Pat. No. 5,559,367 discloses the use of diamond-like carbon to electrically insulate levels within semiconductors. U.S. Pat. No. 4,809,876 discloses polymeric beverage containers utilizing a coating of diamond-like carbon to reduce the gas and vapor permeability through the container.
Polymeric films, such as those used in flexible circuits, are typically electrically insulating and exhibit a propensity towards the generation of static electrical charges (referred to hereinafter as static charges). Static charges are undesirable for a number of reasons. Contaminants such as dust particles are attracted by static charges. Static charges are also known to damage various types of electronic devices, such as semiconductor devices, due to electrical currents associated with the discharge of accumulated static charges. For example, flexible circuits are now used in many hard disk drives for providing electrical connection between the actuator arm and the head assembly. The rapid dissipation of static charges (hereinafter referred to as static discharge) during assembly of the flexible circuit to the head assembly is one of the major sources of failure. The heads in disk drives can be destroyed by relatively small amounts of current associated with static discharge.
Therefore, what is needed is a flexible circuit construction that permits static charges to be controllably and reliably dissipated from a surface of the circuit such that the potential for damage from static discharge to electrical components connected to the circuit is reduced.